This invention relates to coupling units for adjacent sections of transmission lines from which radiation, or wave energy, is coupled, more particularly to such couplers for adjacent sections of coaxial cable and it is an object of the invention to provide an improved coupling unit of this nature.
Intrusion detection systems intended for use as perimeter surveillance systems associated with walls or fences that surround the area to be protected have included single lengths of conductors or cables. Such systems are described in the application Ser. No. 6/172,944, referred to above. A coupling unit for adjacent sections of transmission line, or coaxial cable, particularly for installations wherein a relatively large number of sections of coaxial cable and concomitant large numbers of coupling units was not an element of the prior systems.
The benefits of low cost, simplicity in assembling and efficiency in operation are achieved in the transmitter, coaxial cable sections, receiver, discrete radiating antennas and coupling unit system as described in the indicated application Ser. No. 6/172,944. The coupling unit for use in such a system must, likewise, be of low cost, simple to assemble and efficient in operation.
At the frequencies used in relevant intrusion systems attenuation is an important factor and must be reduced. Regular television coaxial cable having 75 ohm characteristic impedance is a good solution for this and has the advantage of low cost. The coupling units for sections of such cable must have low attenuation or, insertion loss, and enable impedance matching to take place, not only between adjacent sections of coaxial cable but from the coaxial cable to the radiating antennas.
In the latter aspect, the coupling unit requires an opening through which the wave energy is coupled from the center conductor of the coaxial cable. The increase in series impedance of the cable because of this opening must be compensated for to avoid impedance mismatching. In the coupling unit the coupling coefficient, or degree of coupling, to the radiating antenna, as by an intervening transmission line can be selected to give optimum transmission of the transmitted signal as well as to the reflected signal resulting from the movement of an intruder into the protected perimeter.
The coupling unit desirably should lend itself to ready assembly at the factory to an antenna unit for elimination of field assembly problems. Similarly, it should enable the field connectability of adjacent coaxial cable sections with a minimum of effort and without any introduction of further losses.
It is a further object of the invention to provide an improved coupling unit having the advantages and requirements as indicated above and as described in the following summary of the invention.